Combustion apparatus with gas filtering and heat exchanging means

ABSTRACT

The invention relates to an apparatus for burning fossil fuel, for instance oil, gas, coal, biogas formed from organic waste, or the like, which apparatus comprises combustion means for burning this fuel in addition to converting means for converting the energy becoming available in this combustion into a desired form of energy, for instance a burner which can transfer the combustion heat to a heat exchanger for heating of heating medium, air or mains water, which apparatus further comprises a supply conduit for combustion air and a discharge conduit for flue gases. In order to increase the efficiency of such an apparatus the apparatus according to the invention has the special feature that the supply conduit for outside air and the discharge conduit for flue gases are mutually communicating via a water vapour and heating exchanging system such that at least a part of the water vapour present in the flue gases is transferred to the incoming combustion air, with the vapour pressure difference between both substance flows as driving force for the water vapour exchange.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an apparatus for burning fuel, for instanceoil, gas, coal, biogas formed from organic waste, or the like in whichthe apparatus comprises a combustion means for burning this fuel inaddition to a converting means for converting the energy becomingavailable during this combustion into heat, cold or into a combinationof thermal energy with power. For instance, a burner which can transferthe combustion heat to a heat exchanger for heating of a heating medium,such as air, tap water, or a combustion motor, for the purpose ofheat/power generation, in which the apparatus further comprises a supplyconduit for combustion air and a discharge conduit for flue gases.

2. Description of the Prior Art

Such an apparatus is known in diverse embodiments, for instance, heatingfurnaces for heating room areas either indirectly via water as heatingmedium or by direct heating of air, for private use or for professionalpurposes, for heating of tap water and the like.

Another example of such an apparatus is a thermally driven absorptionheat pump or cooling installation for heating and/or cooling purposes.

Yet another example of such an apparatus is a heat/power installation,wherein the heat from the cooling water and the flue gases is used forheating purposes and the power is used to generate electricity by meansof a generator or to drive one or more heat pumps.

The object of the invention is to provide an apparatus which is adaptedrelative to the prior art such that its efficiency is considerablyincreased.

Forming the basis of the invention to be described hereinafter is theinsight that it is possible to increase the efficiency of knownapparatus by reducing the latent and sensible enthalpy in the flue gasesof such a known apparatus.

It is known that for instance the flue gas of natural gas-fired heatingfurnaces contains considerable amounts of so-called latent energy in theform of water vapour. In gas-fired, non-condensing so-called "improvedefficiency furnaces", wherein the flue gas has a temperature between130° C. and 230° C., the loss of latent heat content via the flue gasesamounts to roughly 9.5% and of sensible heat content is between 6% and9%. The total efficiency of these furnaces lies between 80% and 85%. Atan air excess of 10% each 10K temperature fall in flue gas ofnon-condensing furnaces implies an increase in efficiency of about 0.5%.

The energy yield of gas-fired heating furnaces can thus be considerablyincreased by further cooling of the flue gas to below the dew-point ofthe flue gases. This dew-point depends on the air excess that isapplied. At an air excess of for instance 1.27 the dew-point lies atroughly 53° C.

Such condensing heating installations which, when properly embodied,have using efficiencies of over 90% of the upper calorific value ofnatural gas are called high efficiency (HE) installations. The desiredtemperature decrease of the flue gases is achieved by enlarging the heatexchanging surface between the flue gases and the medium to which heatis relinquished. In central heating systems for house heating anadditional so-called flue gas condenser was originally added for thispurpose to the already present heat exchanger, and later both wereintegrated into one heat exchanger. The low temperature of the fluegases also required an additional flue gas vent for discharge of thesegases.

A drawback of this known method is that the efficiency gain to be madein this manner is limited and greatly determined by the installedheating surface, because condensation of the flue gases only occurs at areturn water temperature lower than 50° C., and the installed heatingsurface in dwellings is designed at a feed/return water temperatureusually of 90/70° C. at maximum heat demand. The actual return watertemperature will thus lie above the dew-point temperature for aconsiderable part of the heating season, wherein no condensationtherefore occurs and efficiency lies between 80 and 86%. For CentralEuropean conditions a seasonal efficiency of only about 90% is thereforeachieved.

SUMMARY OF THE INVENTION

In view of the above, the apparatus according to the present inventionhas a gas separating membrane including a gas separating layer facingthe flue gases and as part of a water vapour and heating exchangingsystem adapted for exchanging heat between the combustion air and theflue gases. Preferably, the apparatus has a gas separating layercomprised of polydimethylsiloxane. It is also preferable that the gasseparating layer is applied to a microporous carrier comprised of apolysulfon.

An even greater increase in efficiency is obtained with an apparatuswhich has the special feature in which additional humidified combustionair is fed into the water vapour exchanging system. This additionalmoist air flow can come from elsewhere in the vicinity and be added tothe combustion air humidified by the water vapour exchanging system.Both air flows are supplied jointly to the combustion apparatus.

The apparatus according to the invention can, in addition to the abovestated burner, also used for instance in a combustion motor for combinedgeneration of heat and mechanical energy. In all cases the heat ispreferably used below a temperature of about 82° C. for reasons whichwill later become apparent, particularly with reference to FIG. 2.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be elucidated with reference to the annexeddrawings. In the drawings:

FIG. 1 shows graphs in which the condensate percentage of the chemicallyformed water and the condensate gain relative to a known high efficiencyfurnace are shown as a function of the flue gas temperature;

FIG. 2 shows a graph in which the efficiency of six different centralheating furnaces, including a furnace according to the invention isshown as a function of the average water temperature;

FIG. 3 shows a very schematic view of an apparatus according to theinvention;

FIG. 4 shows a partly broken away perspective view of a membrane modulewhich can serve to transport moisture from the flue gases to theincoming ambient air in the apparatus according to FIG. 3;

FIG. 5 shows another type of water vapour exchanger in a highlyschematic view;

FIG. 6 is a greatly simplified schematic view on very large scale of agas absorption membrane;

FIG. 7 is a simple block-diagrammatic view of the moisture recirculationmaking use of a gas absorption membrane; and

FIG. 8 shows a simple block-diagrammatic view of an energy generatingdevice for heat, cold or heat/power.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1 two quantities are plotted against the flue gas temperature,expressed in °C. The left-hand vertical axis shows the condensate as apercentage of the chemically formed water, while the right-hand verticalaxis relates to the condensate gain compared to a high efficiencyfurnace according to the prior art at an air excess of 1.27. In thisrespect reference will also be made hereinafter to FIG. 2. In the graphsare drawn arrows to the left and to the right. These arrows indicate towhich quantity the relevant graph relates. Line 1 relates to theefficiency of a condensing high efficiency furnace of the prior art withsupply of dry combustion air. Line 2 refers to an apparatus according tothe invention with an air excess of 1.1 and an isothermic moistureextraction from the flue gases of 90%. Line 3 likewise relates to anapparatus according to the invention, but now with an air excess of1.27, likewise with a moisture extraction percentage from the flue gasesof 90%. Line 4 shows the difference between lines 3 and 1. It is notedin this respect that line 1 intersects the horizontal axis at atemperature of about 53° C., so that at this temperature line 4coincides with line 3. From that point the graph 4 can be seen ascontinuing along the line 3, which for this purpose also carries theindication 4'.

The great dependence of the extent of condensation and thus of theefficiency on the flue gas temperature is illustrated in FIG. 1, whichshows, at a determined air excess and as a function of the flue gastemperature, the amount of condensate as a percentage of the amount ofchemically formed water vapour.

This strong dependence of the energy efficiency on the flue gastemperature and thus on the water temperature is typical of condensingheating installations and does not occur in non-condensing heatinginstallations such as for instance in so-called improved efficiency (IE)heating installations.

FIG. 2 shows the efficiency of six different types of heating furnacesof the 90/70 system as a function of the average heating watertemperature. All furnaces are measured under the same conditions.

The bottom line 51 relates to an older central heating furnace, that is,a furnace from before about 1981.

The following line 52 relates to a comparatively modern central heatingfurnace, that is, a furnace from after about 1981.

The following line 53 relates to a normal furnace of the type withimproved efficiency.

The following line 54 relates to a so-called furnace of the type withsuper-improved efficiency.

The following line 55 relates to a condensing high-efficiency furnace ofa type supplied by applicant.

The following line 56 relates to a furnace according to the inventionwithout further cooling of flue gases but with improved condensation.

The top line 57 relates to a furnace according to the invention withimproved condensation and simultaneous cooling of the flue gases.

According to FIG. 2, the line 55, for HE furnaces at 10° C. higher watertemperature the efficiency is in the order of 1% lower at temperaturesabove 50° C. to 2-3% at lower temperatures. It can also be seen that theefficiency of heating installations of the type with improved efficiencyis practically constant. The drawback of a limited efficiency for roomarea heating applies to an even greater extent for mains water heatingwherein no condensation at all occurs. In so-called HE combi-furnaceswherein mains water heating is provided by a hot-water storage furnaceheated indirectly via the HE furnace, appliance efficiencies of onlyabout 60% are achieved. Since the energy consumption for mains waterheating rises relatively when compared to room heating as a result ofthe increasing degree of insulation of dwellings, steps to increase theefficiency for mains water heating are becoming of increasingly greaterimportance.

Other drawbacks to the present HE heating installations are:

1) In respect of the low temperature of the vapour-saturated flue gases,condensation water is formed in the outlet system. This aggressive watercan have an adverse effect on brickwork outlet ducts. In order toprevent this expensive metal flue pipes must be placed in such ducts. Inorder to prevent this, HE heating Installations are often placed inattics, wherein use is made of separate plastic roof ducts;

2) when there is frost, dangerous ice formation may occur on the outletor chimney wherein this can break off and there exists in any case thedanger of falling lumps of ice;

3) a visually disturbing condensation plume and, in the case ofhorizontal wall ducts, a possible adverse effect on surroundingmaterials by the condensation.

In order to obtain a better heat recovery it would seem self-evident tocause the in and outgoing air flows to heat exchange with each other incounterflow. The enthalpy absorption capacity of the ingoing air flow ishowever too small in relation to the enthalpy decrease of the flue gasesas soon as condensation occurs in the heat exchanger. Example: a fall inthe temperature of a quantity of flue gas from 60° C. to 5° C. resultingfrom the combustion of 1 m³ natural gas with an air excess of 1.27releases about 690+5000=5690 kJ sensible plus condensation heat, whilethe cold ingoing air flow, when heated from 5° C. to 60° C., can onlyabsorb 740 kJ or about 13%. The absorption capacity of the ingoing airis thus too small for a good heat recovery.

The invention is based on the insight that a much better heat recoveryis possible if the dew-point temperature of the flue gases weresuccessfully increased in the combustion space to above the return watertemperature. This should take place by bringing about, in an apparatusor exchanger, moisture and heat exchange, if possible simultaneously,between the combustion air and the flue gas in counterflow. For a goodheat and moisture transfer the dividing wall between both air flows inthis apparatus must possess the following properties:

1. a large contact surface

2. a low thermal resistance

3. a large permeability to water vapour compared to other components inthe air flows.

FIG. 3 shows a heating furnace 5 which via a gas feed pipe 6 receivescombustion gas 7. Via an air feed pipe 8 an internal burner alsoreceives combustion air 9. The burner (not drawn) can heat water 11received via a feed pipe 10 and deliver heated water 13 via an outletpipe 12. The heating furnace 5 is of the condensing type. Condensation15 can be discharged via a drain pipe 14. Heated and watervapour-containing flue gases 17 can be discharged via a flue gasesoutlet pipe 16. Situated between the air feed pipe 8 and the flue gasesdischarge pipe 16 is a membrane module 18 which can extract water vapourfrom the flue gases 17 and supply it to the combustion air 9. The watervapour transport is indicated with arrows 19. Heat exchange between theflue gas and the combustion air also takes place in the membrane module18 simultaneously with the moisture exchange if the combustion air issupplied at a lower temperature than the flue gas temperature.

In this apparatus 20 little or no condensation will take place in eitherair duct if the temperature change keeps pace with the humidity change.Should condensation nevertheless occur, the released heat will not bewholly lost but partly contribute to heating of the combustion air. Thedriving forces behind heat and moisture exchange are the temperaturedifference and the vapour pressure difference between both air flows atany moment during passage through the apparatus.

It can be easily appreciated that the moisture content of the combustiongases will, due to moisture recirculation, exceed the maximum even up to80° C. flue gas temperature, whereby condensation will occur. Withoutmoisture recirculation the moisture content in the furnace is constantbecause the outgoing moisture flow is the same as the chemically formedmoisture flow. By recirculating moisture, the moisture content in thefurnace will rise and thus also the outgoing moisture flow. Due to therising vapour pressure difference the returned moisture flow willlikewise begin to increase whereby the moisture content of the furnacerises further. It is easily appreciated that the moisture content of thefurnace will approach exponentially a determined final value which isreached when the difference in outgoing and ingoing moisture flow is thesame as the chemically formed moisture flow. This final value will onlybe reached, however, if at the prevailing water return temperature therelative humidity of the outgoing flue gases remains below 100%. Asimple example shows that this is not however the case, even at 80° C.flue gas temperature.

EXAMPLE

Conditions:

moisture recirculation 90%

gas supply 1 m³ /h

air excess 1.27

moisture content combustion air 0%

temperature flue gas and combustion air 80° C.

RH combustion air 0%

RH flue gas before module 100% or 48 kPa at 80° C.

chemically formed water vapour 1.684 m³ /h

Calculated values:

air flow rate before exchanger:

10.71 m³ /h, no water vapour

air flow rate after exchanger:

18.65 m³ /h, 7.94 m³ /h water vapour at 42.6 kPa

flue gas flow rate before exchanger:

18.98 m³ /h, 8.99 m³ /h water vapour at 48 kPa

flue gas flow rate after exchanger:

11.04 m³ /h, 1.05 m³ /h water vapour at 9.6 kPa

It can be seen from this table that the difference in ingoing/outgoingmoisture flow equals 8.99 m³ /h-7.94 m³ /h=1.05 m³ /h. This is smallerthan the chemically formed water vapour flow of 1.684 m³ /h. Thedifference of 0.634 m³ /h or 37.6% is thus separated off as condensateflow. This implies a rise in efficiency by 0.376×9.5%=3.6%.

In similar manner the condensate flow for other flue gas temperaturesand air excesses can be calculated as a percentage of the chemicallyformed water vapour. The results of isothermic flue gas moisturerecirculation are shown in FIG. 1. At temperatures slightly above 80° C.the condensation is found to have decreased to zero: with moisturerecirculation the dew-point of the flue gases has risen from 53° C. toabout 82° C. Also shown in FIG. 1 on the right-hand vertical axis is thegain in recovered latent heat as a percentage of the chemically formedwater compared to HE furnaces without moisture recirculation. Thereoccurs an asymmetrical curve with a maximum gain of about 96% at theoriginal dew-point of 53° C. where HE furnaces stop condensing. Even at80° C. flue gas temperature, 38% is still separated.

The figure takes no account of the fact that the situation under partialload is even more favourable due to the over-dimensioning of the module.Under partial load conditions wherein the flow rates and the flue gastemperatures are lower, the degree of humidification will rise above 90%and the condensate flows will be higher.

In mains water heating, wherein as a result of the high flue gastemperatures at full load conditions and high furnace water temperatureswithout moisture recirculation, condensation never occurs, thecondensate flow increase according to FIG. 1 is always large. Through anabsolute decrease of the sensible losses (120° C. to 80° C.) by 2% andthrough an absolute decrease of the latent losses by 4 to 5% the mainswater efficiency can therefore rise in total by 7%.

FIG. 1 assumes an ambient air with a relative humidity of 0%. If air at20° C. and 50% RH is drawn in for instance from the dwelling, thedew-point then rises without moisture recirculation from 53° C. to 54.7°C. The moisture content of the combustion air rises by about 7.4%. TheHE furnace will at the same flue gas temperature below 53° C. separateall this additional moisture so that below 53° C. an efficiency gain ofabout 0.7% is achieved at any flue gas temperature. This effect alsooccurs with moisture recirculation, but now up to the dew-point of 82°C., but to a much lesser extent and increasingly less as the moisturetransfer proceeds better. At a constant assumed moisture transferefficiency of for instance 90% only 10% of the already present moisturecontent of the combustion air will be separated as condensate, at 80%efficiency 20% will be separated and so on.

The utilization of already present moisture in the intake air over thewhole temperature range through which the installation operates makesadvantageous the utilization of the latent enthalpy of low temperaturemoisture sources during application of moisture recirculation. Thisapplies to both room area heating in the winter and mains water heatingin the winter and in the summer when the moisture content of outside airis several times higher than in the winter. Particularly in the case ofmains water heating with flue gas temperatures in the vicinity of 80°C., wherein according to FIG. 1 there is a strong variation in thecondensate flow, increase of the dew-point by several degrees is veryimportant. In addition to condensation of ambient moisture, inparticular the separation of the chemically formed moisture also risesby of tens of percent. It can also be seen from the figure that thesmallest possible air excess is herein very favourable.

As ambient moisture sources, in addition to outside air, which can becombined advantageously with the moisture recirculation system can bementioned:

pre-humidified air and pre-heated outside air. The humidifying/heatingsystem can be a sunlight collector with external heat supply or a groundheat and moisture exchanger;

warm and moist ventilation air from dwellings and buildings;

warm and moist air from processes such as for instance used steam.

With moisture recirculation the temperature of the outgoing flue gas canlie just above the return water temperature as a result of the greaterair flow rates and the improved heat transfer in the furnace due to heatradiation and condensation. The decrease in the flue gas temperaturesfrom for instance 120° C. to 80° C. produces a further increase in theefficiency of about 2 percent points. Since it was assumed that theexchanger simultaneously transfers heat in addition to moisture, theflue gas temperature after the heat exchanger will also have fallen tofor instance 20° C. above ambient temperature. The total efficiency canrise in this manner to above 98%. If herein condensation occurs eitherin the flue gas duct or in the combustion air the condensation heat willstill contribute partially to the further heating of the combustion air,respectively to an increase in the efficiency.

In simultaneous exchange of moisture and heat the temperaturedifferences between flue gas and combustion air must be smaller tobetter the moisture exchange proceeds, in order to prevent prematurecondensation occurring. At a moisture transfer efficiency of forinstance 90%, the temperature difference between both mass flows may notamount at any location to more than about 2° C.

The radiation losses, which in a well designed HE furnace amount toroughly 0.5%, will likewise decrease with moisture recirculation due tothe lower temperatures in the combustion space.

The result of the stated effects is that with moisture recirculation bysimultaneous moisture and heat exchange the sensible flue gas sidelosses with ambient air as combustion air can be reduced to roughly 1%(flue gas 20° C. above the ambient air) and the latent losses likewiseto about 1%. With about 0.5% radiation loss a gas-side efficiency of97.5% is thus possible related to the upper calorific value of naturalgas. When additional "free" moisture sources in the environment areutilized the efficiency can be further increased, eventually to above100%. The efficiency improvement is relatively the largest for mainswater heating since this also involves a considerable rise in thechemically formed condensate.

Due to the drying action of the exchanger the problem of thecondensation plume into the environment will be greatly reduced and, atlower flue gas temperatures, even disappear completely. The criterionherefor is whether the connecting line between two points in the Mollierdiagram, wherein the one point represents the moisture condition of theflue gas and the other the moisture condition of the ambient air, liespartly in the mist range below the saturation curve. The greater thepart lying in the mist range, the greater the chance of condensationplume formation. At for instance an air excess of 1.27, 90% humidifyingof the combustion air and a flue gas temperature of 60° C., the moisturecontent of the outgoing flue gas is only 7.5% RH and moisture cannotoccur at any outside air condition, even after cooling of the flue gasin the exchanger to for instance 20° C., wherein the RH rises to about50%.

A favourable effect on the NO_(x) also occurs. It can be seen from thetable that the flue gas flow rate prior to the heat exchanger amounts to18.98 m³ /h. in stoichiometric conditions and without moisturerecirculation this is only 9.39 m³ /h, or 50%. The theoretical flametemperatures will hereby fall from about 2000° C. to 1400° C., wherebythe NO_(x) production will decrease. This is comparable to the decreasein NO_(x) when the air excess rises. Because at flue gas temperaturesbelow 80° C. condensate always occurs, the NO₂ portion will decreasestill further due to absorption by water.

Flue gas moisture recirculation also wholly or partially obviates theother drawbacks such as the dependence on the design of the heatdistribution system, the expensive chimney ducts, the limited placingoptions, the disturbing condensation plume and the danger of iceformation. In order to be able to embody the invention it is thusnecessary to have available equipment which selectively extracts watervapour from the exhaust gas flow and feeds this to the intake air andwhich therein also provides beforehand or simultaneously the necessaryheating of the intake air.

Reference is now made to FIGS. 4, 5, 6 and 7. In the embodiments showntherein use is made of so-called membranes. Using membrane separatingprocesses having as driving force the difference in water vapourpressure, it is possible to selectively extract water vapour from theflue gas flow and relinquish it to the intake air flow. The enormouscontact surface formed by thin membrane walls as separating medium alsoenables a good heat exchange.

An embodiment of the exchanger for performing the said method makes useof gas separating membranes. These are non-porous polymer membraneswhich can have a very high permeability for particular gases, whileother gaseous components in the gas flow are not, or practically not,allowed through. In order to perform the said method a polymer must beselected with a high permeability for water vapour. Use is preferablymade herein of polydimethylsiloxane for the gas separation applied topolysulphon as microporous carrier and formed to hollow fibres. FIG. 5shows a possible embodiment. It is the object to place the flue gas flowand the combustion feed air into direct contact with each other incounterflow in a membrane module with a gas separating membrane asseparating wall. Under the influence of the vapour pressure differenceas driving force the water vapour from the flue gas flow is transportedby means of dissolving and diffusion in the gas separating membrane tothe side of the combustion air feed and carried to the heating device.Condensation of the water vapour in the flue gas flow respectively thecombustion air flow can occur at higher flue gas temperatures. This canbe prevented by preheating the combustion air. It has been found fromexperiments with the said material that the matter transfer coefficientfor water vapour in moist and dry air flows which exchange moisture incounterflow does not vary significantly with a variation of thetemperature and the relative humidity of the air. This is a greatadvantage in dimensioning the exchanger in different temperature andmoisture conditions.

The embodiment according to FIG. 4 makes use of two transverselyapproached hollow-fibre membrane modules with gas absorption membranesmanufactured from a large number of thin, hollow, microporoushydrophobic polymer fibres. Polypropylene, polyimide or polyethersulphonis preferably used for this purpose. The module 21 shown in FIG. 4comprises a housing 22 with two end spaces 23, 24 which are respectivelyconnected to a flue gas intake 25 and a flue gas discharge 26. Thespaces 23 and 24 are mutually connected via tubular membranes 27. Thethrough-flow of ambient air is drawn with arrows 28. This ambient airflows along the membranes 27 and can thus carry with it the water vapourtransported therethrough extracted from the flue gases, as shown as 19in FIG. 3. Both for the intake air flow and the outlet air flow amembrane module is optionally provided with different dimensions. Thefibres are approached transversely by the air flow. Flowing through thefibres as absorbent is a watery liquid with hygroscopic properties. Themicroporous membrane wall serves as separating medium between the liquidand the air and is permeable for gases but not for the liquid and has avapour pressure for water vapour lying between the vapour pressures ofthe intake air and the outlet air. In preference ethylene glycols or abrining liquid can be used as hygroscopic liquid. The hygroscopic liquidcirculates between both modules, wherein, in counterflow to the airflow, moisture is extracted from the outlet air and relinquished to theinlet air. Reference is made to FIGS. 6 and 7.

FIG. 6 shows schematically a micro-view of a gas absorption membrane.This membrane 31 can transport moisture from gas passing by as accordingto arrow 34 via continuous openings 33 and deliver it to an absorbingagent flowing past on the other side as according to arrow 35.

FIG. 7 shows the arrangement schematically. The moisture-absorbing andalso heat transferring medium circulates in the circuit 36, 37. The fluegases and the outside air are designated respectively 17 and 9 asaccording to FIG. 3.

During the absorption process of the water vapour by the absorbent, theheat of evaporation is converted into absorption heat which isdistributed through the air and the absorbent depending on therespective mass flow rates. The effect is that both mass flows areincreased in temperature. When the mass flows for liquid and air areroughly equal and at a specific heat of the absorbent which is about1000× greater than for air, the temperature increase of the outgoingoutlet air is limited to a few degrees, while that of the absorbent canrise to more than 10° C. depending on the water content of theabsorbent. Practically all condensation heat will then be released tothe absorbent. In the second module the absorbent subsequentlyrelinquishes water vapour to the dry intake air by means of desorptionand cools again herein to the outlet air temperature. Pre-heating of theintake air can take place if desired by embodying the first part of thedesorption module with non-porous membrane fibres. In the case ofinsufficient desorption further heating of the absorbent can be realizedif desired with the heating installation, for instance by heat exchangewith the feed conduit to the water distribution system. Duringabsorption the vapour pressure of the absorbent lies, depending on thewater content, at 7 to 12 kPa (water content 9% respectively 19%) at avapour pressure in the air flow of 20 kPa. The vapour pressuredifference is thus in the order of 10 kPa. It has been found fromearlier tests with a module of 400 cm² that the moisture transport at avapour pressure difference of 10 kPa amounts to about 3.7 mg/s and thatwith fibres with enhanced matter transfer properties at least doublethis, or 6.4 mg/s, is possible. In the above stated example of a 20 kWheating installation the moisture flow for discharging amounts to 0.84g/s (corresponding with about 2 kW power). A 130 times larger module, ora module of about 5 m², is thus necessary for this purpose.

This new principle of heat recovery by flue gas moisture recirculationcombined with sensible heat recovery in an exchanger can in principle beapplied to all types of energy generating installations for heat, coldor heat/power combined and fired with fossil fuel or with biogas andequipped with a flue gas heat exchanger and irrespective of the power.FIG. 8 shows as example the schematic diagram of a heat/powerinstallation for combined generation of heat and power. Via a fan 41outside air is drawn in via the water vapour exchanger 18 and suppliedin pre-heated and humidified state to a combustion motor 42 which alsoreceives gas via a gas feed conduit 43. This combustion motor 42, whichcan be embodied as gas motor, gas turbine, hot air motor or othersuitable motor, converts the supplied energy partly into mechanicalenergy and partly into heat. The mechanical energy is taken off viashaft 47 and fed for instance to a generator for generating electricityor to a compressor of a cooling or heat pump installation. The heatcomes from the motor cooling and from the flue gases of the combustionmotor and is fed to for instance a central heating system 48. The fluegas heat exchanger 46 provides cooling of the flue gases. Via aso-called "eco" 46, flue gases 17 are guided along the moistureexchanger 18 and are then discharged. The moisture recirculation willagain result in a greatly increased moisture content of the combustionair and therefore to a greatly increased dew-point. Via the "eco" orflue gas condenser 46 the condensate is separated off in the same manneras described above for HE central heating installations. The abovementioned moist air flows 49 coming from elsewhere in the environmentcan also be fed to the combustion air to further increase the dew-pointtemperature and the condensate flow. It makes no difference herein inparticular whether the power generating unit makes use of externalcombustion, such as for instance in steam furnaces, absorption coolingmachines or heat pumps or Stirling machines, or of internal combustionsuch as in for instance gas motors or gas turbines. It is known that theNO_(x) emission in gas motors and gas turbines can be reduced bydecreasing the combustion temperatures using water or steam injection.This effect can likewise be achieved using the principle of flue gasmoisture recirculation without adding water externally but by making useof chemically formed water. Thus created is a kind of dry "low NO_(x) "burner. Particularly in the case of gas turbines, which are verysensitive to contaminants in the water due to possible calciumprecipitation on the turbine blades, where a very large investment isentailed in demineralizing the feed water. Using moisture recirculationthe desired humidifying is provided by the chemically formed, clean andfree water vapour from the energy generating installation itself. Theuse of membranes as moisture/heat exchanger for instance preventsrecirculation of particles from the flue gas flow as a result of thespecific separating process.

Finally, it is possible to combine the exchanger for moisturerecirculation and heat recovery with that of flue gas washer to furtherpurify the flue gases of NO_(x) and/or SO₂. Use can advantageously bemade herein of the above described microporous polymeric and hydrophobicgas absorption membranes as so-called membrane stripper. The absorbentmust then contain in addition to a moisture absorbing agent componentswhich specifically remove NO_(x) and SO₂.

With respect to oil or coal-fired installations it will be apparent thatcooling of the flue gases to below 80° C. implies that the dew-point ofSO₂ of 160° C. is also not attained. The economiser or flue gascondenser will of course have to be suitable for this purpose.

The share of latent energy of the chemically formed water vapour, in thetotal released energy during combustion of fossil fuels or biogas formedfrom organic waste, forms the difference between the upper calorificvalue or combustion efficiency and the lower calorific value or netheating value of the relevant fuel. Broadly speaking, this difference is10% in the case of natural gas and biogas, 6% in the case of oil and 3%in the case of coal.

I claim:
 1. An apparatus for burning fossil fuel, which apparatuscomprises:combustion means for burning the fuel; converting means forconverting the in energy produced in the combustion into a desired formof energy; a supply conduit for combustion air; a discharge conduit forflue gases; and a water vapour exchanging system having a membrane,wherein the supply conduit for combustion air and the discharge conduitfor flue gases are mutually communicating via the membrane in the watervapour exchanging system such that at least a part of the water vapourpresent in the flue gases is transferred to the incoming combustion air,with a vapour pressure difference between the flue gases and combustionair acting as a driving force for water vapour exchange, wherein themembrane is a gas separating membrane having a gas separating layerfacing the flue gases, and wherein the water vapour exchanging system isalso adapted for exchanging heat between the combustion air and the fluegases.
 2. The apparatus as claimed in claim 1, wherein the gasseparating layer includes polydimethylsiloxane.
 3. An apparatus forburning fossil fuel, which apparatus comprises:combustion means forburning the fuel; converting means for converting the energy produced inthe combustion into a desired form of energy; a supply conduit forcombustion air; a discharge conduit for flue gases; and a water vapourexchanging system having a membrane, wherein the supply conduit forcombustion air and the discharge conduit for flue gases are mutuallycommunicating via the membrane in the water vapour exchanging systemsuch that at least a part of the water vapour present in the flue gasesis transferred to the incoming combustion air, with a vapour pressuredifference between the flue gases and combustion air acting as a drivingforce for water vapour exchange, wherein the membrane is a gasseparating membrane having a gas separating layer facing the flue gases,wherein the water vapour exchanging system is also adapted forexchanging heat between the combustion air and the flue gases; andwherein the gas separating layer is applied to a microporous carrierincluding a polysulfon.
 4. The apparatus as claimed in claim 1, whereinadditional humidified combustion air is fed to the water vapourexchanging system.